#### Answer

$18\sqrt 2$

#### Work Step by Step

We first multiply the separate square roots together to create one square root. Then, we break the square root into its factors, and finally, we simplify the factors that are perfect squares:
$2\sqrt 3 \times 3\sqrt 6$
=$(2\times3)\times(\sqrt 3\times\sqrt 6)$
=$(6)\times(\sqrt {3\times6})$
=$(6)\times(\sqrt {18})$
=$(6)\times(\sqrt {2\times9})$
=$(6)\times(3\sqrt {2})$
=$18\sqrt 2$